Television camera including an image isocon tube



Oct. 7, 1969 COPE ETAL 3,471,741

TELEVISION CAMERA INCLUDING AN IMAGE zsocou TUBE Filed April 7, 1967 VENTORS Armin/v.0. OPE,

A T TOINEY United States Patent O i US. Cl. 315-11 7 Claims ABSTRACT OF THE DISCLOSURE In a photoemissive type of pickup tube having a target and an electron multiplier, a return beam reflected by the target includes both specularly oriented and scattered electrons. In image isocon operation, the scattered electrons constitute a signal and are fed through an opening in a separating electrode and passed into the electron multiplier. The specularly oriented electrons are intercepted by the separating electrode. In order to deflect the return beam before it reaches the separating electrode, so that only the scattered electrons pass through the opening therein, four steering coils are positioned outside of the tube envelope in a plane adjacent to the separating electrode. Desired deflection of the beam in this plane is accomplished by controlling the direction and magnitude of D.C. electric currents fed to the steering coils.

BACKGROUND OF THE INVENTION Field of the invention Our invention relates to a television camera having an image isocon tube in which both specularly oriented and scattered electrons are reflected from a target, and particularly concerns an improved means for separating the scattered electrons from the specularly oriented reflected electrons.

Description of the prior art An image isocon pickup tube comprises an elongated envelope having an electron gun and an electron multiplier at one end thereof, and a photoemissive cathode at the other end. Intermediate the electron gun and the photoemissive cathode is a storage target electrode. The storage target electrode is exposed on one side to photoelectrons from the photocathode, and on the other side to an electron beam from the electron gun. Between the storage target and electron gun is positioned a separating electrode for separating two components of the portion of the electron beam returning to the electron gun. The separating electrode collects the returning beam component consisting of specularly reflected electrons, and allows the component consisting of scattered signal electrons to enter the electron multiplier. Positioned between the separating electrode and the storage target electrode is a steering electrode system comprising four electrodes for so modifying the orientations of the specularly reflected electrons and scattered electrons in the beam, that the separating electrode may accurately separate these components of the return beam. A pickup tube of this type is described in US. Patent No. 3,225,237 issued Dec. 21, 1965, to A. D. Cope.

While a steering system is desirable for separating in space the reflected electrons from the scattered electrons so that substantially only the specularly reflected electrons may be intercepted by the separating electrode, it is accompanied by several problems.

' One of these problems is related to the operation of the tube. Prior steering systems of the four electrode type described in the aforementioned Cope patent, have Patented Oct. 7, 1969 relied on an electrostatic field produced by four electrodes for contributing to the steering function. The voltage impressed on each of the four electrodes, however, has been found to require critical adjustment for proper operation of the tube. This requirement has added appreciable complexity in setting up the tube for operation, as compared with a conventional image orthicon. This complexity is responsible, at least in part, for failure of the image isocon tube to find wider application in the pickup tube field.

Another problem involves structural considerations. The provision of four steering electrodes, such as the type of electrodes described in the Cope patent referred to, requires a complicated internal structure of the tube, thereby adding to its cost of fabrication.

Furthermore, the use of four steering electrodes within the envelope of the tube requires four added contact prongs in the tube base for energizing the four electrodes. The provision of such four added contact prongs is accompanied by the disadvantages of complicating the base structure and introducing hazards of shorts between two adjacent prongs operated at a relatively high voltage difference, because of the relatively small spacing permitted between such adjacent prongs.

The type of image isocon structure referred to gives rise to a very serious problem when it is desired to employ a tube of this type in interchanged relation with respect to an image orthicon tube. In this situation, the increased number of contact prongs of the image isocon tube rendered necessary because of the four internal steering electrodes, would render this type incapable of accommodation by an image orthicon socket. Furthermore, the presence of the type of steering electrodes within the tube envelope required for isocon operation, would introduce problems of interference in image orthicon operation.

SUMMARY OF THE INVENTION In a television camera having an image isocon tube, a separating means is disposed within the envelope of the tube and a steering means is located outside of the tube envelope. The steering means may comprise four coils spaced around the outer surface of the tube envelope in a plane intermediat that of a separating electrode and a charge storage electrode within the tube. The coils may be operated in two pairs, each pair constituting two coils disposed on opposite sides of the tube envelope. By varying the amplitude and direction of D.C. electric current for each of the pairs, it is feasible to adjust the magnitude and direction of a magnetic steering field in a region adjacent to the separating electrode.

The magnetic steering means is relatively easy to adjust to position the return beam for desired separation of the reflected and scattered electrons therein. Furthermore, the disposition of the steering means externally of the tube envelope, simplifies the tube and base structures and contributes to feasibility of interchange of image isocon and image orthicon tubes in a television camera.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of a television camera in accordance with our disclosure;

FIG. 2 is a sectional View taken along the line 22 of DESCRIPTION OF THE PREFERRED EMBODIMENT The television camera shown in the figures comprises a photoemissive pickup tube 12, a lens 14, and a coil system including a focusing coil 16, deflection or scanning coils 18, and tube alignment coils 20. In addition and in accordance with the present disclosure, there is provided a steering coil system comprising four coils 22, 24, 26, 28 shown in FIG. 2, positioned between the deflecting coils 18 and the alignment coils 20.

The tube 12 comprises an elongated glass envelope 30 having an electron gun 32 and an electron multiplier 34 positioned in one end portion thereof. The electron gun 32 includes a beam-forming aperture plate 36 having a relatively small opening of about 0.002 inch diameter, and a thermionic cathode 38. The components of the electron gun and multiplier may be of standard construction.

The tube 12 at its other end terminates in a glass faceplate 40 having on the inner surface thereof a conventional semitransparent photocathode 42 comprising, for example, a commercially available S-10 photocathode including silver, bismuth, oxygen and cesium. Spaced from the photocathode 42 is a storage target 44 which may comprise any conventional structure such as a thin membrane of glass, magnesium oxide, or aluminum oxide, supported in a conventional manner. A decelerator mesh screen 46 is closely spaced from the gun side of the storage target 44. A secondary electron collector mesh screen 48 is closely spaced from the photocathode side of the storage target 44. Photoemission from the photocathode 42 forms an electrostatic image on the target 44 which corresponds to a light image focused on the photocathode by the lens 14. In view of the thinness of the target 44 the electrostatic image formed thereon appears on both sides of the target. For focusing the image information in the photoelectron emission from the photocathode 42 upon the target 44, focusing means are provided such as a metal ring 50 adjacent to the photocathode and a metal cylinder 52 supporting the mesh screen 48 and the target 44.

An electrode 54, which may be a conducting coating on the inner wall of the tube envelope 30 extends from a region close to the mesh screen 46 to a region of a plane including the steering coils 22, 24, 26, 28. The electrode 54 supplements the focusing function of the focusing coil 16, with respect to an electron beam produced by electron gun 32, and a return beam reflected by target 44.

During operation, the cathode 38 of the electron gun 32 may be grounded and tube elements such as the photocathode 42, the mesh screen 46, the ring electrode 50, the cylindrical electrode 52 and the wall electrode 54, may be impressed with the potentials shown in FIG. 1. A scanning beam from the electron gun 32 passes through the mesh screen 46 and is decelerated to a few volts of energy as it approaches the target 44. The decelerated beam is scanned across the surface of the target 44 facing the electron gun 32, by the scanning coils 18, and establishes a stable potential on the gun side of the target 44 that is relatively close to the potential of the cathode 38. Any change in the stable potential, such as that produced by an electron charge pattern from the photocathode 42 corresponding to a light image focused on the photocathode by the lens 14, will be sensed by the scanning beam. The sensing is assomplished by a different response of the beam to non-charged regions of the target from that to regions charged substantially above cathode potential. Such response modifies the character of the return beam. The return beam from the non-charged regions consists solely of specularly reflected electrons. The return beam from charged regions of the target consists of two parts. One part comprises specularly reflected electrons and the other part comprises scattered electrons. The scattered electrons are characterized by a range of velocities that is different from that of the specularly reflected electrons. The specularly reflected electrons and the scattered electrons, by virtue of their different velocities, will assume different locations in the return beam 56. Thus, as shown schematically in FIG. 1, a first portion 57 of the return beam is formed by specularly reflected electrons While a second portion 60 is constituted of scattered electrons. The scattered electrons forming return beam portion 60 are employed in the isocon as the signal electrons representative of the electrostatic image formed on target 44.

For collecting and dissipating the specularly reflected electrons 57 and separating them from the signal electrons 60, there is provided a separating electrode 62 having a central opening 64. The separating electrode 62 is positioned so that its opening 64 is near the plane of an antinode of the return beam 56. Such positioning is desirable since the scattered electrons are characterized by maximum lateral displacement from the specularly reflected electrons at such antinode. Such appreciable radial displacement facilitates collection of the specularly reflected electrons by the separating electrode 62, while it permits the bulk of the scattered signal electrons to pass through the opening 64 and into the electron multiplier 34. A persuader electrode 66 is provided between the separating electrode 62 and the electron multiplier 34 and is connected to a suitable potential source for urging the electrons passing through the opening 64 of the separating electrode, into the electron multiplier.

In order to insure that substantially only the specularly reflected non-signal electrons are intercepted by the separating electrode 62, improved means are provided for steering the portion of the return beam 56 adjacent to the separating electrode 62, so as to more effectively space the scattered electrons 60 from the specularly reflected electrons 56 and from the edge of opening 64 in the separating electrode 62. The improved means may comprise two pairs of coils 22, 24 and 26, 28 as shown in FIGS. 2 and 3. The coils in each pair are disposed adjacent to diametrically opposite outer sides of the tube envelope and the several coils are equidistantly angularly spaced around the tube envelope as shown in FIGS. 1 and 2. When D.C. electric current of suitable magnitude and direction is fed to the steering coils referred to, a magnetic field is produced within the tube envelope that effects lateral displacement of the return beam 56 adjacent to the separating electrode 62, into a position wherein substantially all of the scattered electrons 60 are free to enter the opening 64 in the separating electrode 62 and to pass into the electron multiplier 34, while specularly reflected electrons only, travel in a path for interception by the separating electrode 62.

The operation of the four steering coils in accordance with the present disclosure is illustrated in FIG. 3. An electron return beam 56 moving into the paper is shown initially appreciably laterally displaced from the separating electrode opening 64. A magnetic field indicated by lines 74 having a direction denoted by the arrowheads on the field lines and produced by electric currents traveling through the coils in the directions of arrows 76, 78, will deflect the beam 56 in a direction normal to the field lines and towards the opening 64. For accomplishing the desired deflection the magnetic field strength may be from to 75 gauss produced by an electric current through each coil group of about milliamperes. As shown in the example of FIG. 3, the magnetic field lines under these conditions extend in a direction about 45 from the axis of each of coil groups 22, 24 and 26, 28. It will be noted that this direction of the magnetic field lines is perpendicular to the direction in which the return beam 56 should be moved in order to secure that type of register with the opening 64 in the separating electrode, that is required for image isocon operation. Such normal relationship between the magnetic lines of force and the direction in which the return beam should be moved for desired register, will cause the beam to move in the lastnamed direction.

Where the return beam 56 is initially closer to the opening 64 in the separating electrode 62 than as shown in FIG. 3, a magnetic field of relatively low intensity will be sufficient to deflect the return beam into the desired registry shown in FIG. 1, and wherein the specularly oriented electrons 57 are intercepted by the separating electrode 62 and the scattered electrons 60 pass into the opening 64.

Prior to adjustment of the magnitude and direction of electric current passing through the coils 22, 24, 26, 28 in a steering operation, the position of the return beam 56 with respect to the opening 64 cannot be determined by direct visual inspection. However, the current output from the electron multiplier 34 provides an indication as to the position of the return beam 56. Thus, when no current flows from the tube output, i.e., from the electron multiplier '34, an indication is provided that no portion of the beam 56 is in register with the opening 64. This is the initial condition shown in FIG. 3. By adjusting the current input to each of the two coil groups shown, the magnetic field may be rotated. When, during such rotation the magnetic field assumes the position shown in FIG. 3, with respect to return beam 56, the output from the tube will increase since the beam will be moved into at least some register with the opening 64 and therefore, some electrons will reach the electron multiplier 34. Since this indicates that a correct angular positioning of the magnetic field has been realized with respect to the direction of displacement of the return beam, the coils are then kept at the adjustment producing such increased output. Any increase in the current fed into the coils for full deflection of the return beam into the desired register with opening 64, is kept at a value so as to preserve the ratio between the currents in the two coil groups required for preserving the aforementioned angular position of the magnetic field. Thus, if a stronger magnetic field should be desired for securing the desired register, the ratio of the increased currents fed to the two coil groups should be the same as that prior to the increase. The electric currents fed to the two coil groups may be increased, while preserving the aforementioned ratio, until the output of the tube is at a maximum with a minimum of noise.

The magnetic type of steering effected by the magnetic field produced by coils 22, 24, 26, 28, is appreciably easier to control than the electrostatic steering means heretofore available in the art. Furthermore, the magnetic steering means disclosed herein can be positioned effectively outside of the tube envelope, thereby reducing impediments to use of the tube as either an image orthicon or an image isocon. Such interchangeability may be desirable in a situation wherein the tube may be called upon to function in association with either an existing image orthicon circuit or an image isocon circuit. In this situation, the interchangeability of the tube, permits a reduction in the number of spare tubes normally kept on hand. Furthermore, the elimination of four internal electrodes heretofore required for electrostatic steering, results in a reduction in the cost of fabrication of the tube. Another important feature resides in the fact that the use of external magnetic steering means permits survival of the steering means when the tube becomes inoperative. These features introduce appreciable economy in a camera in which the tube is used.

While the steering coils 22, 24, 26, 28 may have any suitable structure for performing the desired steering function, an exemplary structure thereof is shown in FIGS. 1 and 2. Each steering coil may comprise a pole piece 68 made of cold rolled steel, for example, and having a diameter of about inch. The pole piece is sungly embraced by an annular trough 70 made of an insulating material such as Teflon. Within the trough 70 are confined about 700 turns of insulated wire 72. The wire 72 may be made of copper and may have an outside diameter of about 7.4 mils. When the steering coils 22, 24, 26, 28 are operatively mounted adjacent to the outside surface of the tube envelope 30, the coil axes are in perpendicular relationship to the longitudinal axis of the tube envelope, as shown in FIG. 1.

We claim:

1. A television camera comprising:

(a) a pickup tube having:

(1) an elongated envelope,

(2) a photocathode in one end of said envelope,

(3) an electron multiplier in the other end portion of said envelope,

(4) a target electrode in said envelope in a plane intermediate said photocathode and said electron multiplier, and

(5) a separating electrode within said envelope and in a plane intermediate said target electrode and said electron multiplier, and

(b) means disposed outside said envelope for producing a magnetic field within said envelope between the planes of said separating electrode and said target, for steering a return electron beam from said target in a path wherein specularly reflected electrons of said return beam are intercepted by said separating electrode and electrons scattered from the target in said return beam are directed into said electron multiplier.

2. A television camera according to claim 1 and wherein said means for producing a magnetic field comprises a plurality of coils positioned outside of said tube and means for electrically connecting said coils to electric current sources of predetermined magnitude and direction.

3. A camera tube adapted to be selectively operated in the image isocon mode and in the image orthicon mode, comprising:

(a) a charge storage target,

(b) a photocathode disposed to one side of said target,

(c) a plurality of elements progressively spaced from the other side of said target, said elements in the order of progression from said target comprising:

(1) a focusing electrode,

(2) a separating electrode,

(3) an electron gun for directing an electron beam to said target, and

(4) an electron multiplier,

(d) said separating electrode having a relatively large opening for selectively passing an entire return beam from said target in image orthicon operation, and for intercepting specularly reflected electrons, and passing scattered electrons in said return beam into said electron multiplier in image isocon operation, and

(e) external magnetic means for steering said return beam in image isocon operation for maximum interception of said specularly reflected electrons and for maximum passage of said scattered electrons through said opening,

(1) said external means being removable for image orthicon operation of said tube.

4. A camera tube in accordance with claim 3 and wherein said external means comprises a plurality of coils for generating a magnetic field extending into said envelope in a region between said focusing electrode and said separating electrode.

5. A television camera comprising:

(a) a pickup tube of the image orthicon type having an elongated envelope containing:

(1) an electron gun for producing an electron beam,

(2) a target for reflecting a portion of said beam,

(3) a focusing electrode for focusing said return beam, and

(4) a separating electrode, and

(b) a coil system positioned outside of said envelope and comprising:

(1) alignment coils coextensive With a portion of said electron gun,

(2) deflecting coils spaced from said alignment coil axially of said envelope and coextensive with a portion of said focusing electrode, for deflecting said electron beam, and

(3) steering coils intermediate said alignment coils and said deflecting coils for producing a magnetic field adjacent to said separating electrode for steering said return beam into operative position with respect to said separating electrode.

6. In combintion:

(a) a pickup tube having therewithin:

( 1) a separating electrode,

(2) electron beam return means and an electron multiplier,

(b) magnetic means outside of said tube and coextensive with a region adjacent to said separating electrode for steering said return beam to dispose specularly reflected electrons in said return beam in a path for interception by said separating electrode and to pass scattered electrons in said return beam into said electron multiplier.

7. The combination of claim 6 and wherein said means outside of said tube comprises two pairs of coils spaced around said tube and adapted to produce a steering magnetic field within said tube in a region adjacent to said separating electrode.

References Cited 10 UNITED STATES PATENTS 2,545,982 3/1951 Weimer 315-11 2,579,351 12/1951 Weimer 315-11 X 2,747,133 5/1956 Weimer 315 11 3,158,778 11/1964 Johns 315- 11 15 3,183,400 5/1965 Jensen et a1. 315 11 3,225,237 12/1965 Cope 313-65 RODNEY D. BENNETT, JR., Primary Examiner 20 BRIAN L. RIBANDO, Assistant Examiner US. Cl. X.R. 313-85 

